HIV entry is mediated by the viral envelope glycoprotein, which comprises non-covalently associated surface (gp120) and transmembrane (gp41) subunits. gp120 is primarily involved in recognition of cellular receptors, while gp41 directly mediates membrane fusion. When peptides isolated from the gp41 N- and C-peptide regions (N- and C-peptides) are mixed in solution, they form a six-helix bundle, which represents the post-fusion gp41 structure. Three N-peptides form a central parallel trimeric coiled coil (N-trimer) surrounded by three antiparallel helical C-peptides that nestle into long grooves between neighboring N-peptides. The importance of this structure is indicated by the dominant negative inhibition of HIV entry by N- and C-peptides.
The available inhibitory and structural data support a working model of HIV membrane fusion (FIG. 1). Initially, gp120 interacts with cellular CD4 and a chemokine coreceptor (typically CXCR4 or CCR5), causing large conformational changes in gp120 that propagate to gp41 via the gp41-gp120 interface. gp41 then undergoes a structural rearrangement that unleashes its N-terminal fusion peptide, which embeds in the target cell membrane. At this stage of fusion, gp41 adopts an extended “prehairpin intermediate” conformation that bridges both viral and cellular membranes and exposes the N-trimer region. This intermediate is relatively long-lived (minutes), but ultimately collapses as the N- and C-peptide regions of each gp41 monomer associate to form a hairpin structure. Three such hairpins (trimer-of-hairpins) form the 6-helix bundle, which forces the viral and cellular membranes into tight apposition and leads to membrane fusion. This structure likely corresponds to the core of the fusion-active state of gp41 and shows similarity to the proposed fusogenic structures of envelope fusion proteins from influenza, Moloney Murine Leukemia Virus, and simian immunodeficiency virus (SIV), and Ebola virus.
According to this model, an inhibitor that binds to the N-trimer and prevents hairpin formation can inhibit viral entry. This has been well supported by the discovery of numerous peptide, protein, and small molecule inhibitors that bind the N-trimer. A particularly interesting feature of the N-trimer is the deep hydrophobic “pocket” formed by its 17 C-terminal residues. This pocket has several enticing features as an inhibitory target including: (1) a very highly conserved sequence, (2) an essential role in viral entry, (3) a compact binding site vulnerable to inhibition by short peptides, and (4) the availability of several designed peptides (e.g., IQN17, IZN17, 5-helix, NCCGN13 that authentically mimic the pocket structure). What is needed in the art are peptides with suitable pharmacokinetic properties that can inhibit the entry of gp41 into cells.